an igneous, volcanic rock, of intermediate composition, with
aphanitic to porphyritic texture.
Its mineral assembly
is usually quartz and plagioclase. Biotite, hornblende and pyroxene
are common accessory minerals. Alkali feldspar is
Classification of andesites may be refined
according to the most abundant phenocryst.
Example: olivine andesite, if olivine is the principal
accessory mineral. Andesite can be considered as the extrusive
equivalent to plutonic diorite. As diorites, they are characteristic
of subduction tectonic environments in active oceanic margins,
such as the western coast of South America.
The name andesite is derived from the Andes mountain
Andesitic magma in island arc regions (i.e. active oceanic
margins) comes from wet melting of mantle wedge peridotite.
When an oceanic plate is subducted, it contains a lot of water.
This water is removed from the subducting slab because of the
increasing pressure and temperature. Flow in the mantle wedge
carries the water down to sites directly below the volcanoes,
where the water enables the melting of the mantle peridotite.
The initial melt is usually of basaltic composition.
On its way to the surface, the melt stalls and cools, enabling
the fractional crystallization of silica poor
minerals, thus raising the silica content of the remaining melt
and resulting in andesitic magma.
The Andesite Line is the most
significant regional distinction in the Pacific. It separates the deeper,
basic igneous rock of the Central Pacific Basin from the partially submerged
continental areas of acidic igneous rock on its margins. The
Andesite Line follows the western edge of the islands off California
and passes south of the Aleutian arc,
along the eastern edge of the Kamchatka Peninsular, the Kuril
Islands, Japan, the Mariana Islands,
the Solomon Islands,
and New Zealand. The dissimilarity continues northeastward along
the western edge of the Albatross Cordillera along South America
to Mexico, returning then to the islands off California. Indonesia,
the Philippines, Japan, New Guinea, and New Zealand—all
eastward extensions of the continental blocks of Australia and
Asia—lie outside the Andesite Line.
Within the closed loop
of the Andesite Line are most of the deep troughs, submerged
volcanic mountains, and oceanic volcanic islands that characterize
the Central Pacific Basin. It is here that basaltic lavas gently
flow out of rifts to build huge dome-shaped volcanic mountains
whose eroded summits form island arcs, chains, and clusters.
Outside the Andesite Line, volcanism is of the explosive type,
and the Pacific Ring of Fire is the world's foremost belt of explosive